uefi: serial: fix core::fmt::Write impl

The old implementation may cause partial writes - although on real
hardware Status::TIMEOUT errors are not the exception but also appear
on the happy path. It is more correct to just retry.

The underlying implementation has built-in endless loop protection.

Author: phip1611

uefi-test-runner/src/main.rs

@@ -10,6 +10,7 @@ extern crate alloc;
 
 use alloc::string::ToString;
 use alloc::vec::Vec;
+use core::fmt::Write;
 use uefi::mem::memory_map::MemoryMap;
 use uefi::prelude::*;
 use uefi::proto::console::serial::Serial;
@@ -115,7 +116,10 @@ fn send_request_helper(serial: &mut Serial, request: HostRequest) -> Result {
     serial.set_attributes(&io_mode)?;
 
     // Send a screenshot request to the host.
-    serial.write(request.as_bytes()).discard_errdata()?;
+    //serial.write(request.as_bytes()).discard_errdata()?;
+    serial
+        .write_str(request.as_str())
+        .expect("should write all bytes");
 
     // Wait for the host's acknowledgement before moving forward.
     let mut reply = [0; 3];

uefi/CHANGELOG.md

@@ -43,6 +43,7 @@
 - **Breaking:** Renamed `DevicePathNode::to_string()` to `DevicePathNode::to_string16()`
   to better differentiate with the new `to_string()` coming from the new
   `Display`.
+- Fixed potential partial writes in `fmt::Write` impl for `Serial` protocol
 
 # uefi - v0.36.1 (2025-11-05)
 

uefi/src/proto/console/serial.rs

@@ -10,6 +10,7 @@ use crate::proto::unsafe_protocol;
 use crate::{Error, Result, ResultExt, Status, StatusExt, boot};
 use core::fmt;
 use core::time::Duration;
+use log::error;
 use uefi_raw::protocol::console::serial::{
     SerialIoProtocol, SerialIoProtocol_1_1, SerialIoProtocolRevision,
 };
@@ -384,7 +385,16 @@ impl Serial {
 
 impl fmt::Write for Serial {
     fn write_str(&mut self, s: &str) -> fmt::Result {
-        self.write(s.as_bytes()).map(|_| ()).map_err(|_| fmt::Error)
+        // We retry on Status::TIMEOUT but propagate other errors
+        self.write_exact(s.as_bytes()).map_err(|e| {
+            let msg = "failed to write to serial device";
+            // Simple check to prevent endless recursion if a logger
+            // implementation uses the serial protocol
+            if !s.starts_with(msg) {
+                error!("{msg}: {e}");
+            }
+            fmt::Error
+        })
     }
 }